The PC-to-PC voice feature can work behind most types of NATs. These are the NATs Yahoo! Messenger supports:

1. Coned NAT - common in home networks
2. Full Cone NAT - common in home networks
3. Port Restricted Cone NAT - common in corporations
4. Symmetric NAT - common in corporations (most restrictive type of NAT)

Note, however, we cannot guarantee that the PC-to-PC voice feature will work on every single NAT.

Definitions

Restricted Cone NAT: A network address translator where all requests from the same internal IP address and port are mapped to the same external IP address and port. Unlike a Full Cone NAT, with this NAT an external host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X.

Full Cone NAT: A network address translator where all requests from the same internal IP address and port are mapped to the same external IP address and port. Furthermore, any external host can send a packet to the internal host by sending a packet to the mapped external address.

Port Restricted Cone NAT: A network address translator like a Restricted Cone NAT, but the restriction includes port numbers. Specifically, an external host can send a packet, with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P.

Symmetric NAT: A network address translator where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port. If the same host sends a packet with the same source address and port, but to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host.